Chapter 15Autonomic Nervous System & Visceral Reflexes

• General properties of the autonomic nervous system

• Anatomy of the autonomic nervous system

• Autonomic effects on target organs

• Central control of autonomic function

General Properties of the ANS

• Motor nervous system that controls glands, cardiac and smooth muscle – also called visceral motor system

• Regulates unconscious processes that maintain homeostasis– blood pressure, body temperature, respiratory airflow

• ANS carries out its actions without our intent (automatically)– biofeedback techniques

• training that teaches some people to control hypertension, stress and migraine headaches.

Visceral Reflexes

• Unconscious, automatic responses to stimulation of glands, cardiac or smooth muscle

• Receptors detect internal stimuli -- stretch, blood chemicals, body temperature, etc.

• Afferent neurons connect to interneurons in the CNS• Efferent neurons carry motor signals to the effectors

– ANS is the efferent neurons of these reflex arcs

– glands, smooth or cardiac muscle is the effectors

• ANS modifies effector activity rather than causing it– high blood pressure activates a visceral baroreflex

Visceral Reflex to High Blood Pressure

• High BP detected by arterial stretch receptors, signal transmitted to CNS, efferent signals travel to the heart, heart slows reducing BP

• Separate reflex arc for low BP exists

Divisions of the ANS

• Two divisions that innervate the same target organs• Two divisions may have cooperative or contrasting

effects• Sympathetic division prepares body for physical

activity– increases heart rate, BP, airflow, blood glucose levels, etc

• Parasympathetic division has calming affect on many body functions & assists in bodily maintenance– digestion and waste elimination

• Autonomic tone is the normal rate of activity that represents the balance of the two systems

• Effects of each depend upon neurotransmitters released

Somatic versus Autonomic Pathways

ANS = 2 neurons span the distance from CNS to effectors• presynaptic neuron cell body in CNS -- brain or spinal cord

• postsynaptic neuron cell body in peripheral ganglion

• Origin of presynaptic neurons– lateral horns of gray matter of thoracic to lumbar cord (T1-L2)

• Sympathetic chain ganglia (paravertebral)– 3 cervical, 11 thoracic, 4 lumbar, 4 sacral & 1 coccygeal ganglia– white & gray communicating rami suspend ganglia from spinal

nerve– variety of pathways of preganglionic fibers

• enter ganglia & synapse• travel to higher or lower ganglia & synapse• pass through chain without synapsing to reach collateral ganglia via

splanchnic nerves

• Neuronal convergence – each postganglionic cell receives synapses from multiple preganglionic cells– produces widespread effects on multiple organs

Sympathetic (thoracolumbar) NS

Efferent Pathways of Sympathetic NS

The Sympathetic Chain Ganglia

Pathways of Preganglionic Sympathetic Fibers

Collateral Ganglia & Abdominal Aortic Plexus

Summary of Sympathetic Innervation

• Effectors in the body wall are innervated by sympathetic fibers found in spinal nerves

• Effectors in head and thoracic cavity are innervated by fibers in sympathetic nerves

• Effectors in abdominal cavity are innervated by sympathetic fibers in splanchnic nerves.

Adrenal Glands

• Paired glands sit on superior pole of each kidney• Cortex

– secretes steroid hormone

• Medulla– modified sympathetic ganglion that secretes neurotransmitters

(hormones) into blood and not onto other neurons• catecholamines (85% epinephrine & 15% norepinephrine)

• Sympathoadrenal system is the closely related functioning adrenal medulla and symphathetic NS

Parasympathetic (craniosacral) NS• Origin of preganglionic fibers

– pons and medulla oblongata for cranial nerve nuclei– spinal cord segments S2-S4

• Pathways of preganglionic fibers– cranial nerves III, VII, IX and X

• cardiac, pulmonary, esophageal, abdominal aortic plexus

– arising from sacral spinal cord• pelvic splanchnic nerves & inferior hypogastric plexus

• Terminal ganglia in target organs due to long preganglionic and short postganglionic fibers

Efferent Pathways of Parasympathetic NS

Parasympathetic Functions of Cranial Nerves

• Oculomotor nerve (III) – narrows pupil & focuses lens

• Facial nerve (VII)– regulates secretion of tear,

lacrimal & 2 salivary glands

• Glossopharyngeal (IX)– regulates parotid salivary

gland

• Vagus nerve (X)– muscles and glands of the

viscera as far inferiorly as the proximal half of colon

Enteric Nervous System

• Nervous system of the digestive tract

• Composed of 100 million neurons found in the walls of the digestive tract (no components found in CNS)

• Has its own reflex arcs

• Regulates motility of viscera and secretion of digestive enzymes and acid in concert with the ANS

Neurotransmitters & Receptors

• Types of neurotransmitters released and types of receptors on target cells determines effects of ANS

• Sympathetic NS has longer lasting effects – reaches bloodstream before being broken down

• Many other substances also released as neurotransmitters– enkephalin, substance P, neuropeptide Y, neurotensin, nitric oxide

Cholinergic Receptors for ACh

• Acetylcholine binds to 2 classes of receptors– nicotinic receptors

• occur on all ANS postganglionic neurons, in the adrenal medulla, and at neuromuscular junctions (skeletal muscle)

• excitatory when ACh binding occurs

– muscarinic receptors• occur on all gland, smooth muscle & cardiac muscle cells

that receives cholinergic innervation

• either excitatory or inhibitory when ACh binding occurs due to subclasses of muscarinic receptors

Adrenergic Receptors for NE• Norepinephrine binds to 2 classes of receptors

– alpha adrenergic receptors• NE binding is excitatory

– beta adrenergic receptors• NE binding is inhibitory

• Exceptions to normal results (EPSP or IPSP)– existence of subclasses of each receptor type

• alpha 1 and 2; beta 1 and 2

• Function by means of 2nd messengers– beta receptors activate cyclic AMP, alphas2 receptors

suppress it and alpha1 receptors use calcium

Dual Innervation• Most of viscera receive nerve fibers from both

parasympathetic & sympathetic divisions– antagonistic effects oppose each other

• exerted through dual innervation of same effector cells– heart slowed down or speeded up

• exerted because each division innervates different cells– pupillary dilator muscle & constrictor pupillae change pupil size

– cooperative effects seen when 2 divisions act on different effectors to produce a unified effect(salivation)

• parasympathetic NS increases salivary serous cell secretion• sympathetic NS increases salivary mucous cell secretion

• Both divisions do not normally innervate an organ equally

Dual Innervation of the Iris

Control Without Dual Innervation

• Adrenal medulla, arrector pili muscles, sweat glands & many blood vessels receive only sympathetic fibers

• Sympathetic tone is a baseline firing frequency• provides partial constriction called vasomotor tone

– increase in firing frequency = vasoconstriction– decrease in firing frequency = vasodilation

• Vasomotor tone can shift blood flow from one organ to another according to changing needs– sympathetic stimulation increases blood to skeletal &

cardiac muscles -- reduced blood to skin

Sympathetic Tone and Vasomotor Tone

Sympathetic division prioritizes blood vessels to skeletal muscles & heart in times of emergency.

Blood vessels to skin vasoconstrict to minimize bleeding if injury occurs during stress or exercise.

Central Control of Autonomic Function• ANS is regulated by several levels of the CNS

– limbic system connected to hypothalamus• pathway through which emotions influence ANS

– hypothalamus (major visceral motor control center)• nuclei for primitive functions – hunger, thirst, sexuality

– reticular formation & brainstem nuclei• can respond directly to sensory input from cardiac,

vasomotor, & GI tract

– spinal cord reflexes• defecation & micturition reflexes are integrated in the

spinal cord• brain can inhibit these responses consciously

Drugs and the Nervous System• Sympathomimetics enhance sympathetic activity

– stimulate receptors or norepinephrine release

• Sympatholytics suppress sympathetic activity– inhibit norepinephrine release or block receptors

• Parasympathomimetics enhance activity while Parasympatholytics suppress activity

• Management of clinical depression– Prozac blocks reuptake of serotonin to prolong its mood-

elevating effect

– MAO inhibitors interfere with breakdown of monoamine neurotransmitters

• Caffeine competes with adenosine (inhibitory causing sleepiness) by binding to its receptors